The presently disclosed embodiments relate to ink jet architectures for high speed production printing, and more particularly, production piezo ink jet (PIJ) architectures employing a large array of printheads in a direct to web architecture; however, the embodiments also apply to other modular jet architectures producing print by employing a large array of printheads.
A piezo ink jet printhead will expel a volume of ink upon an ink chamber contraction resulting from an applied voltage. Normally the ink has to be heated to a comparatively high temperature because the ink will be solid at room temperature. In a production printing embodiment, 20 or more printheads are configured in an array with each printhead having several hundred jets. Because all jets must be working at the same time, reliability requirements for the printheads are compounded. In other words, the need to mitigate disruptions associated with jet failures is critically important in any production printing embodiment that employs large numbers of non-redundant jets.
Printheads experience a jet failure whenever any of their jets are either not jetting enough ink or not jetting any ink at all. Some jet failures are intermittent, which means the corresponding jets either spontaneously recover, or are recovered by a maintenance procedure. Other jet failures are chronic, which means the function of the corresponding jets cannot be recovered. When a jet fails, it is not known if the failure is chronic until several attempts to recover the jet have failed. The process for attempting to recover failed jets is a fairly involved. A relatively large volume of ink is forced through the head in an effort to purge whatever is blocking the failing jet. The face of the printhead is cleaned with an automatic wiper to remove excess ink. One can imagine that for a large roll of paper comprising a production web, if a jet recovery operation had to occur every time any one of the substantial number of printheads failed, then the purge operation would be very disruptive to the extent that no reasonable commercial operation could result. Nevertheless, jet failures have to be dealt with, and in a typical production environment, operators may frequently be faced with an uncomfortable trade-off between choosing the direct cost of changing one or more printheads versus dealing with the potentially time consuming disruption of performing printhead purge and maintenance cycles, and the additional trouble shooting procedures in the printer to recover one or more printheads. When a failing printhead has to be “swapped” with a replacement head, a “cold swap” is performed so that the system cannot return to a production ready state until the replacement unit and the delivery ink are heated to a print-ready production status. Post production sorting and recovery of jobs and pages with image defects is also problematic.
One option for meeting customer requirements with production ink jet architectures is to target only the portion of the market that has high tolerance for jetting faults, in other words, has consistently low print quality requirements. Fortunately, acceptable print quality is not a strict function of perfect jet performance, but rather also depends on the combination of: (1) specific print images within a job, (2) location and degree of jet failures, and (3) job specific customer print quality requirements. What is needed is a set of methods to support a business friendly strategy for practicing intervention-by-choice printhead maintenance that can help operators to comprehend how the print system's present jet performance is likely to impact specific jobs, and then re-optimize scheduling of print jobs and maintenance interventions accordingly.
Enabling elements for these methods could include: jet performance monitoring, automatic or manual print job image characterization, job quality requirement tagging, simulated defect previewing, job re-queuing, web changeover tracking, operator alerts, and a well featured GUI. With these elements properly employed, the system could assume a variable degree of fault tolerance that is compatible with job-to-job dependent customer needs. Not all such elements would be required though to better align job queuing and intervention timing with customer business value. The degree of automation and sophistication could also be variable. For example, a system with 20 partial width heads, each with 1000 jets nominally requires all 20,000 jets to work on demand at all time but many jobs can be printed with one or more weak or missing jets. Customers do not want a system to shut down and be prompted for immediate maintenance when the printer is producing acceptable output.
There is a need for a system that minimizes the impact of jetting errors on the print shop workflow and facilitates an intervention strategy that can adapt system operating capabilities with specific job demands.